Chapter 14 — Solutions
Overview of Chapter 14
• Solutions
• Concentrations:
Molarity
Molality
Mole fraction
• Colligative Properties
Freezing point depression
Boiling point elevation
1
Today’s questions to consider:
• What is the effect of adding a solute on a vapor pressure
curve?
• How much salt do we need to add to ice to melt it?
• How does antifreeze work?
• Osmosis
Today’s Topics
• Colligative Properties
• Raoult’s Law
• Vapor pressure depression
• Boiling point elevation
• Freezing point depression
Colligative Properties
On adding a solute to a solvent, the props.
of the solvent are modified.
• Vapor pressure decreases
• Melting point
decreases
• Boiling point
increases
• Osmosis is possible (osmotic pressure)
They depend only on the NUMBER of solute
particles relative to solvent particles, not
on the KIND of solute particles.
Understanding Colligative Properties
To understand colligative properties, study
the LIQUID-VAPOR EQUILIBRIUM for a
solution.
Understanding Colligative Properties
Look at the liquid-vapor equilibrium for a solution.
Chapter 14 — Solutions
2
Understanding Colligative Properties
VP of H2O over a solution depends on the
number of H2O molecules per solute molecule.
Raoult’s Law
An ideal solution is one that obeys Raoult’
Raoult’s law.
PA = XA • PoA
Psolvent proportional to Xsolvent
Psolvent = Xsolvent • Posolvent
VP of solvent over solution
= (Mol frac solvent)•(VP pure solvent)
= RAOULT’S LAW
Vapor Pressure Lowering
Because mole fraction of solvent, XA, is always
less than 1, then PA is always less than PoA.
The vapor pressure of solvent over a solution is
always LOWERED!
Raoult’s Law
Assume a solution containing 62.1 g (1.00 mole)
of ethlene glycol in 250. g of water is ideal.
What is the vapor pressure of water over the
solution at 30°C?
(The VP of pure H2O at 30°C is 31.8 mm Hg)
Solution
Xglycol = 0.0672
and so Xwater = ?
Because Xglycol + Xwater = 1
Xwater = 1.000 - 0.0672 = 0.9328
Pwater = Xwater • Powater = (0.9382)(31.8 mm Hg)
Pwater = 29.7 mm Hg
Changes in Freezing and Boiling
Points of Solvent
VP Pure solvent
1 atm
Boiling Point Changes
VP solvent
after adding
solute
P
BP solution
BP pure
solvent
T
The boiling point of a solution is higher than that of the pure
solvent.